fix: Ais destination should be string

Fixes #243.

pgns/129794.js

@@ -20,7 +20,7 @@ module.exports = [
   },
   {
     node: 'navigation.destination.commonName',
-    source: 'Destination'
+    value: n2k => n2k.fields['Destination']
   },
   {
     node: 'design.draft',

test/129794_ais_class_a_static_data.js

@@ -89,4 +89,42 @@ describe('129794 AIS Class A Static and Voyage Related Data', function () {
     //TODO enable when sensors.ais.class is in the schema
     //tree.should.be.validSignalKVesselIgnoringIdentity
   })
+
+  it('Empty destination converts to empty string', () => {
+    const msg = {
+      "prio": 6,
+      "pgn": 129794,
+      "dst": 255,
+      "src": 43,
+      "timestamp": "2014-08-15T19:05:41.030Z",
+      "input": [
+        "2014-08-15T19:05:41.030Z,6,129794,43,255,75,05,b0,7f,79,10,00,00,00,00,45,53,59,32,31,31,31,50,56,4c,2d,31,31,31,20,56,41,50,50,45,52,20,20,20,20,20,20,37,7c,01,3c,00,14,00,b4,00,14,40,00,00,00,00,00,00,40,40,40,40,40,40,40,40,40,40,40,40,40,40,40,40,40,40,40,40,00,e1"
+      ],
+      "fields": {
+        "Message ID": 5,
+        "Repeat indicator": "Initial",
+        "User ID": 276398000,
+        "IMO number": 0,
+        "Callsign": "ESY2111",
+        "Name": "PVL-111 VAPPER",
+        "Type of ship": "Law enforcement",
+        "Length": 38,
+        "Beam": 6,
+        "Position reference from Starboard": 2,
+        "Position reference from Bow": 18,
+        "ETA Date": "2014.11.30",
+        "ETA Time": "00:00:00",
+        "Draft": 0,
+        "Destination": "",
+        "AIS version indicator": "ITU-R M.1371-1",
+        "GNSS type": "undefined",
+        "DTE": "available",
+        "Reserved1": "0",
+        "AIS Transceiver information": "Channel B VDL reception"
+      },
+      "description": "AIS Class A Static and Voyage Related Data"
+    }
+    const delta = mapper.toDelta(msg)
+    delta.updates[0].values.find((pv) => pv.path === 'navigation.destination.commonName').value.should.equal('')
+  })
 })